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(Circulation. 2007;115:1225-1233.)
© 2007 American Heart Association, Inc.

Heart Failure

Muscarinic Modulation of the Sodium-Calcium Exchanger in Heart Failure

Shao-kui Wei, MD; Abdul M. Ruknudin, PhD; Matie Shou, MD; John M. McCurley, MD; Stephen U. Hanlon, MD; Eric Elgin, MD; Dan H. Schulze, PhD; Mark C.P. Haigney, MD

From the Division of Cardiology, Department of Medicine, Uniformed Services University of the Health Sciences, Bethesda, Md (S.W., M.S., J.M.M., S.U.H., M.C.P.H.); Department of Microbiology and Immunology, University of Maryland, School of Medicine, Baltimore (A.M.R., D.H.S.); Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging, Baltimore, Md (A.M.R.); and Division of Cardiology, Walter Reed Army Medical Center, Washington, DC (E.E.).

Correspondence to Mark C.P. Haigney, MD, Division of Cardiology, Department of Medicine, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd, Bethesda, MD 20814. E-mail MCPH{at}aol.com

Received July 25, 2006; accepted December 29, 2006.

Background— The Na-Ca exchanger (NCX) is a critical calcium efflux pathway in excitable cells, but little is known regarding its autonomic regulation.

Methods and Results— We investigated ß-adrenergic receptor and muscarinic receptor regulation of the cardiac NCX in control and heart failure (HF) conditions in atrially paced pigs. NCX current in myocytes from control swine hearts was significantly increased by isoproterenol, and this response was reversed by concurrent muscarinic receptor stimulation with the addition of carbachol, demonstrating "accentuated antagonism." Okadaic acid eliminated the inhibitory effect of carbachol on isoproterenol-stimulated NCX current, indicating that muscarinic receptor regulation operates via protein phosphatase–induced dephosphorylation. However, in myocytes from atrially paced tachycardia-induced HF pigs, the NCX current was significantly larger at baseline but less responsive to isoproterenol compared with controls, whereas carbachol failed to inhibit isoproterenol-stimulated NCX current, and 8-Br-cGMP did not restore muscarinic responsiveness. Protein phosphatase type 1 dialysis significantly reduced NCX current in failing but not control cells, consistent with NCX hyperphosphorylation in HF. Protein phosphatase type 1 levels associated with NCX were significantly depressed in HF pigs compared with control, and total phosphatase activity associated with NCX was significantly decreased.

Conclusions— We conclude that the NCX is autonomically modulated, but HF reduces the level and activity of associated phosphatases; defective dephosphorylation then "locks" the exchanger in a highly active state.

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